AMONG the pigmented seed coat colors in garden peas (Pisum sativum), "Ruby" is unique in that the color characteristic of the mature testa is recognizable in the very young and immature ovules. This color is a rich dark red in the dry testa and appears as a translucent dark red in the immature seed coats. Ruby is distinct from, and is in no way to be confused with the orange brown seed coat color described by Tschermak2, or with the reddish brown discoloration of gray seed coats occurring under certain moisture and temperature conditions.

Literature

Although the variety is an old one and the "Ruby" character is known to several of the students of heredity in peas, no reference to it appears in the literature. White5 does not list it either from references or as a part of his own experimental work. Neither do Sverdrup1 nor Wellensiek4 mention the character in their papers summarizing the knowledge of heredity in the genus Pisum.

Materials and Methods

The materials used in these experiments were obtained from Dr. Orland E. White, to whom the author is indebted, also, for valuable criticisms and suggestions as well as for library facilities. Originally, this variant race came from Vilmorin & Cie, Paris.

In the spring of 1928 several crosses were made between the variety of peas called Ruby and varieties having gray seed coats. Both the Ruby and the gray seed coated strains had been grown as pure cultures for several generations and had bred true for red-purple flower color as well as for the seed coat characters. The variety Ruby also carried the factor F for the specking pattern of the seed coat. This specking pattern does not contrast strikingly with the dark red seed coat and may easily escape notice except on careful inspection. This is particularly true if for any reason the seeds are shriveled or malformed. Several plants were encountered in the F2 populations on which numerous shriveled, poorly filled seeds showed no specks, yet all of the well-filled seeds of these plants showed at least a few dots.

Experimental Results

In the F1 generation, the immature seed showed the Ruby character in a somewhat diluted intensity but still distinct. In the dry testas the seed coats were gray to brownish red (the latter a weathering characteristic of gray seed coats) rather than the usual dark red to maroon color of the homozygous Ruby. The F1 was also specked.

The results of segregation are shown in the F2 distribution given in Table I.

From this table it will be seen that the monohybrid ratio of 277 Gray or non-ruby: 80 Ruby accords fairly well with the calculated expectation of 267.7:89.3. The distribution of speck and non-speck is quite otherwise, the observed numbers being 238:119 or 2:1 against the expected 3:1 F2 frequency of 267.7:89.3.

The difficulties in classification previously mentioned, account for these results, as the inheritance of this pattern has been fairly well investigated. The discrepancy in Table I between the observed and calculated is mainly due to the factors concerned with specking.

In the cultures represented by the first two distributions in Table I, many plants produced only a few seeds and in some cases these were defective. This may have resulted in some plants being classified as nonspeck which, with better seed, would have appeared as speck.

Cross

Pedigree

Observed and
Calculated

Gray
Spk.

Ruby
Spk.

Gray
Non-spk.

Ruby
Non-spk.

I

Ruby x 50

O

52

8

18

8

C

48.4

16.1

16.1

5.3

II

Ruby x 86

O

63

17

49

13

C

79.9

26.6

26.6

8.9

III

Ruby x 161

O

73

25

22

9

C

72.0

24.0

24.0

8.0

Totals

O

188

50

89

30

C

200.7

66.9

66.9

22.3

Discussion

The character Ruby in this discussion is designated Ru. Since the genetics of seed coat color in Pisum is very imperfectly understood and there are many color relations to be worked out, the symbols Ru (Ruby) and ru (non-ruby) are to be regarded as tentative. On this basis the factors of the Ruby strain used in these experiments are indicated by Ru F, the gray seed coat strains as ru f. In the absence of Ru the seed coats are gray. All strains carried the same flower pigment factors. The relation of Ruby to white flower and colorless seed-coat races is unknown, though presumably Ru expresses itself only when the flower pigment factors are present, since this is true of all but one of the previously investigated seed coat colors in Pisum.

Table I, giving the F2 ratios indicates that Ru and F are independently inherited. In cross III of the table, the observed numbers give a very close approximation to a typical 9:3:3:1 ratio. In the other crosses the discrepancies have already been discussed.

TABLE I-Segregation of Ruby=gray, Specked=Non Specked in F2

The most interesting aspect of the Ruby character is its more or less dominant expression in the immature Ru ru seeds and its practically recessive condition in the mature Ru ru seeds. The expression of Gc (Gray seed coat color) in the mature seeds of the combination Ru ru is probably epistatic, since only at maturity does Gc express itself.

Summary

Ruby, a seed coat color of Pisum, appears to be independent in inheritance from specking (F).

Ruby is tentatively designated Ru, its allelomorph ru.

Ru is dominant in the F1 during the immature stages of seed development, but segregates as a "recessive" in the mature seed coats of the F2.

The ruby seed coat color, being visible in the immature seed coats, and masked by the later appearance of the gray character, gives the appearance of a reversal of dominance. The masking of Ruby in mature Ru ru seeds by Gc may possibly be interpreted as epistasis on the part of Gc.